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skyscraper design

...75 metres (250 feet). It was the form of the steel structure used for the Sears (now Willis) Tower in Chicago. Beyond this height there is another system that appears to have a low premium: the
superframe. In this structure much of the building’s gravity load, and therefore its material, is brought to a diagonally braced
superframe tube at the perimeter by interior transfer trusses of...